Pat Miller does a wonderful nonfiction conference in the fall that includes writing for magazines, but she hasn't announced it yet for this fall. You can see the faculty she had last year here: http://www.patmillerbooks.com/faculty.html

Re: Magazine writing in workshops or conferences?

Suggest it to your regional SCBWI group if you have a way to provide feedback on what you are looking for. Our Mid-Atlantic region has done several local NF writing workshops the last few years because several of us have asked for them. They've been quite good and very well attended!